The variable-thickness plate rolling technology was originated in a Japanese steel company, which started to develop and produce the variable cross-section medium-thickness plate in 1978. Subsequently, French and German steel companies developed variable-thickness plates respectively in the 1980s and 1990s. The development of the variable-thickness rolling technology has been mature in the field of rolling mill for medium-thickness plate.
Most of the existing plate mills are single-stand reversing mills. There does not exist problem of the second flow balance in the process of variable-thickness control. The key point of the variable-thickness is to control the lifting or the depressing of roller gap precisely according to the desired shape, so the variable-thickness control technology of the medium-thickness plate has been developed rapidly. With respect to the medium-thickness plate mill, the conventional hot strip mill is designed to achieve the uniform control of the thickness along the full length of the strip steel, without the capability of the variable-thickness control, so it is more difficult to perform the variable-thickness control on the conventional hot continuous rolling mill.
The conventional hot continuous rolling mill performs control based on the second flow balance. To perform variable-thickness rolling on the conventional hot continuous rolling mill, it is not only required to perform the precise control to the roller gap, but also to perform the control of the timing sequence of the lifting and the depressing of the roller gap of each stand, velocity matching of previous and next stands and the loop stability. It is preferable to avoid the influence on the rolling stability due to the imbalance of the second flow during the thickness varying process. Therefore, it is pretty difficult to perform the precise variable-thickness control on the hot continuous rolling mill. On the other hand, restricted by the user's demands, the application area of the strip still produced with the variable-thickness hot rolling method is very narrow, and most of users need the strip still with uniform thickness, so this technology is developed very slowly in the conventional hot continuous rolling mill.